Source : Webster's Revised Unabridged Dictionary (1913)

Clumper \Clump"er\, v. t. [Cf. G. kl["u]mpern to clod. See
   {Clump}, n.]
   To form into clumps or masses. [Obs.]

         Vapors . . . clumpered in balls of clouds. --Dr. H.
                                                  More.